When an integrated circuit device has been designed by integrated circuit design engineers, and patterns of wires, insulation, and semiconductor material have been laid out to implement the integrated circuit device, it is necessary to pattern a set of masks to be used by a foundry to form the layers of material in the integrated circuit device. The mask patterns are typically several times larger than the patterns they will form on the IC device, and lenses reduce the pattern to the size of the IC. The information used to form each of the masks is generated and stored by a computer in electronic form. It can then be placed on a tape, compact disk, or other electronic medium to be communicated to the foundry or mask vendor and used by the foundry or mask vendor to make the successive mask layers. The design database that is stored by the computer is hierarchical, describing, for example, coordinates of lines and widths of the lines, and where the lines are to be located. This database is converted by the mask vendor or foundry to a non-hierarchical format that can be used to control an E-beam machine to write patterns on a layer of chromium or other opaque metal for making the mask. This process of converting from the design database to the mask making database is called fracturing.
In the past, a mask pattern for an entire layer of the IC has been described by data indicating where all the spaces or solid portions are to be located. As integrated circuit devices have become larger and more complex, the amount of data to describe the mask pattern has increased. Also, many large integrated circuit devices have highly repetitive structures. This repetitive nature has been taken advantage of by communicating data to the foundry or mask vendor in a format that has a hierarchy. One set of data will describe a skeleton of the entire layer, leaving blanks at locations where the repeating elements are to be located, and another set will describe the repeating elements in detail only once rather than at every occurrence. When the design data is fractured to produce the mask making database, all copies of the repeating elements must be included, so the mask making database becomes undesirably large.
Another issue must be dealt with when geometries are very small. Modern design rules may allow line widths, transistor channel lengths, and other features having dimensions on the order of 0.13 microns. The wave length of light used to expose photoresist on the IC device layer is on the order of 0.25 microns. With these dimensions, light diffraction is a significant factor and must be taken into account. Light that passes between two closely adjacent lines will diffract and spread before reaching the IC device being exposed. Thus the resulting structure will not meet design rules and will not be a reliable structure. To avoid this effect, line widths on the mask must be adjusted to compensate for diffraction so the resulting IC has the intended line widths and spacings. These diffraction adjustments are called optical proximity correction or OPC. Different portions of the mask must be adjusted different amounts because line densities of adjacent elements are not the same. There is less adjustment needed when line densities are lower.
In the case of repeating elements, line densities at the edges must be adjusted differently depending on what is adjacent to the repeating elements. If a repeating element is to be placed next to a less dense adjacent region, the optical proximity correction at the edge of the repeating element will be less than if the repeating element is placed next to a dense adjacent region. However, the process of OPC produces a database in which the repeating elements are no longer identical. Thus the benefit of storing just one copy of the repeating element is lost.
Further, when the design database is fractured to produce a mask database, all copies of the repeating elements have been included, and the mask database for a complex IC device has become extremely large. It is desirable to reduce the size of very large mask databases for these very complex IC devices.